1. Field of the Invention
The invention relates to a photoelectric measuring head, and more particularly, to a photoelectric measuring head for the contactless scanning of measurement regions.
2. Discussion of the Prior Art
In the graphics industry, photoelectric measuring heads are utilized for recording the reflectances of test regions on a a printed sheet, particularly by the direct-light method. The photoelectric measuring head can, in particular, be designed as a spectral color-measuring head, as a three-range color-measuring head or as a densitometer measuring head. The measurement regions of the printed sheet are, for example, extra co-printed measurement fields which either are arranged in a line in the form of a print-checking strip, or are distributed arbitrarily over the format or around so-called pictorially important locations of the subject. The use of a photoelectric measuring head in the case of printed products which have just been extracted from the printing machine is particularly critical, since the ink or the applied varnish is still fresh, and smudging of the ink or the varnish must be absolutely avoided.
DD 260,032 A1 discloses a spectral measuring head in the form of a densitometer which is displacable via slide guides in the x and y direction and parallel to the plane of a sheet which is resting on a base. Air-outlet orifices, through which compressed air is blown out, are arranged on the underside of the measuring head, in other words on the side which confronts the article to be measured. An air cushion thus builds up and supports the measuring head in suspension above the surface to be measured. Essentially, the measuring head is supported counter to the weight of the latter above the surface to be measured and does not contact the surface, thereby preventing any smudging. Furthermore, the measuring head is vertically displacable and is connected in an articulated manner to the device guiding it in the x-y plane.
In this known measuring head, in order to generate the supporting air cushion, bores, via which the compressed air emerges, are arranged in a uniform distribution along the outer edge of the underside of the measuring head. A problem associated with this type of arrangement is that the supply of compressed air to the corresponding air-outlet bores must be equally distributed to the bores. Essentially, it is necessary to ensure that the same quantity of air per unit time is blown out via each outlet bore. If this is not the case, particularly when this occurs on diametrically opposite sides, a tendency of the measuring head to tilt can arise. The compressed air emerging at each bore flows via a relatively large cross-section at a very high outlet velocity, so that eddies are generated. The eddies of adjacent outlet orifices can influence one another so that, under certain circumstances, the air cushion supporting the measuring head is set into vibration. The distance, between the measuring optics and the surface to be measured must be kept constant. If the separation distance is not maintained at a constant value, the measuring accuracy is impaired. Accordingly, the vibration causes changes in the distance and thereby produces inaccuracies in the measurement.